Biomedical engineering with real-time biological information, such as eye pressure, blood pressure, core body temperature and neural signals, has become useful in the research for identifying genetic variation susceptibility to diseases. Animal-based research result is expected to make a helpful impact in developing new treatment methods for similar human diseases. A miniature and implantable bio-sensing microsystem with wireless telemetry and RF powering is highly desirable to capture accurate bio-signals and information. These CMOS-based devices have additional aspects for managing a large number of channels: signal multiplexing, amplification, A/D conversion and filtering are done on-chip. Developing the micro-system includes sensing methods study for various vital signals. Among the biological signals, eye pressure for Glaucoma patients is one of the more useful vital signals.
Glaucoma, the second leading cause of blindness, is a debilitating disease that affects millions of people. Current data suggests that 60 million people have glaucoma, and that number is to reach 79 million by the year 2020. Several risk factors put you at a disadvantage to contract the disease, including ethnicity, age, and family history. In glaucoma, death of optic nerve cells leads to blindness. Currently, doctors have very few options in detecting glaucoma, and many of them only detect glaucoma after enough damage has occurred and it is too late to prevent the disease. These include visual field testing, Ocular Coherence Tomography, and Intra Ocular Pressure (IOP) monitoring.
The first two determine glaucoma after the damage has been completed, and can be used as an aid to monitor damage that has already been completed. IOP monitoring is the only method in which doctors can prevent the glaucoma from beginning and is the strongest known contributing factor to optic nerve cell death. Although it is clear that high IOP is harmful to the optic nerve, the exact relationships between the kinetics, duration, and magnitude of IOP elevation and optic nerve damage are not known.
Doctors use several methods to examine IOP's glaucomatous effects. First, Goldmann tonometry (the gold standard) is a noninvasive method to measure the pressure inside the eye. Goldmann tonometry uses the Imbert-Fick law that equates pressure based on flattening of the cornea. This leads to misdiagnosis from individual to individual based on physiological characteristics. Studies conducted by Whitacre et al. found several errors occurring from using these indentation, or mathematical computations, to determine IOP. The only viable invasive measurement technique currently available is microneedle cannulation. This process involves inserting a needle, connected to a pressure gauge, into the anterior chamber of the eye and monitoring. This is a very invasive method of monitoring eye pressure, and if a physician needs continuous data, it is infeasible. As IOP changes considerably even within a single day, typical annual or semi-annual measurements at a doctor's visit can be misleading or uninformative.
The mouse is a well-suited mammalian model for deciphering the mechanisms of this complex disease due to the similarity of its well-known genome to the human. N-ethyl-N-nitrosourea (ENU) mutagenesis followed by phenotypic screening is proving to be a fruitful approach to studying glaucoma. In this approach, there is considerable variability in age of onset of IOP elevation, even in mice with the same mutation. Thus, success is limited where IOP measurement data over many months cannot be continuously and remotely assessed. In order to study the mechanism of the disease, continuous monitoring of intraocular pressure (IOP) using an implanted sensor is useful. The diameter of the anterior chamber in a mouse eye is approximately 3 mm and its depth is 75 microns at the edge and 300 microns in the center. However, current IOP sensors are not suitable to be used as a mouse eye implant due to their size. Furthermore, sensors that use near field inductive coupling do not provide the sensitivity and the sensing distance needed for IOP monitoring with a resolution of 1 mmHg from mice which are awake and roaming within a cage. Therefore, a fundamental challenge lies in the creation of a suitably sensitive sensor that will fit in the small space available in the eye of the mouse and provides the needed continuous monitoring and sensing properties for use with mice in typical laboratory cages.
In addition, implantable pressure sensors are useful with regards to other conditions. As one example, the breast implant market is 1-2 billion dollars a year. Half of those sales are for silicone gel implants. The primary concern is that when they rupture, there is no way to detect the rupture without an MRI scan. The FDA recommends all women receive an MRI every 2-3 years at a cost of $1,500-$2,000 each.
As implantable devices become “smart”, using Application Specific Integrated Circuits (ASICs) and other electrical components to sense their surroundings, power consumption becomes an aspect of running these devices efficiently. Currently, there are several methods in which a power source is supplied to implantable devices. One includes the use of high mAh batteries that allow the device to be powered for many years. Second, such as with implantable hearing aids, inductive coupling is used with an external power source coupling energy to the implanted device to stimulate the hair follicles inside the cochlea.
These two powering techniques, although acceptable, have drawbacks. Using a battery is efficient and allows for a long implant life, but once that battery has died a second usually major surgery is required to outfit a patient with a new battery. In the case of a cardiac pacemaker, this is costly and life threatening. Using inductive coupling, a staple in cochlear implants, is efficient in close range to power the device. However, there lies its drawback, that the external device should always be closely located and arranged for proper alignment and powering.
Improvements and alternatives are therefore needed in this field.